Cortical Mechanical Properties Research Articles

Compaction Dynamics during Progenitor Cell Self-Assembly Reveal Granular Mechanics

Summary We study the self-assembly dynamics of human progenitor cells in agarose microwells that are used for production of chondrogenic organoids. Using image analysis on time-lapse microscopy, we estimate the aggregate area in function of time for a large number of aggregates. In control conditions, the aggregate radius follows an exponential relaxation that is consistent with the dewetting dynamics of a liquid film. Upon introducing Y-27632 Rho kinase inhibitor, the compatibility with the liquid model is lost, and slowed-down relaxation dynamics are observed. We demonstrate that these aggregates behave as granular piles undergoing compaction, with density relaxation that follows a stretched exponential. Using simulations with an individual cell-based model, we construct a phase diagram of cell aggregates that suggests that the aggregate in presence of Rho kinase inhibitor approaches glass transition.

Qu Feng Zhi Tong capsule increases mechanical properties of cortical bone in ovariectomised rats

Abstract Backgroud/objective Qu Feng Zhi Tong capsule (QF, Buchang Pharma, Xi'an, Shanxi, China) is officially approved Fufang formula in China for the treatment of rheumatoid arthritis (RA). As a Traditional Chinese Medicine formula, it consists of seven herbs, among which Erodium stephanianum, Dipsacus asper, and Viscum Coloratum were reported to be able to prevent bone loss and promote bone healing. Moreover, osthole from Angelica pubescens exerted anabolic effect on cortical bone by activating β-catenin-bone morphogenetic protein (BMP) signalling. So, we hypothesise that the formula QF has beneficial potential on preventing osteoporosis. Method In this study, high-performance liquid chromatography (HPLC) was used to identify the main constituents in QF, and SD female rats were ovariectomised (OVX) to examine antiosteoporotic efficacy. Thirty OVX rats were orally administrated with QF (162 and 648 ​mg/kg/day, L-QF, and H-QF) or estradiol (1 ​mg/kg/day, E2) for 12 weeks. Sham and OVX control group were treated with vehicle. Results Eleven compounds in QF extract were identified using HPLC, and the sum of their peak areas was covered 44.2% of total peak area of QF extract. As to in vivo results, compared to OVX group, H-QF had significantly higher ultimate load (P ​ Conclusion QF promoted cortical mechanical properties because of its positive effects on cortical thickness, porosity, and skeletal muscle. H-QF increased cortical thickness by promoting bone formation of periosteum and inhibiting bone absorption of endosteum in OVX rats. The translational potential of this article This study provided theoretical basis for QF as a conventional drug in osteoporosis to enhance cortical bone mechanical and avoid long bone fracture. As cortical diaphyseal bone is an important target of RA, this study also provides a possibility that QF is beneficial to RA by its positive effects on cortical bone to prevent bone erosions in RA patients.

Zoledronate and Raloxifene combination therapy enhances material and mechanical properties of diseased mouse bone.

Current interventions to reduce skeletal fragility are insufficient at enhancing both the quantity and quality of bone when attempting to improve overall mechanical integrity. Bisphosphonates, such as Zoledronate (ZOL), are used to treat a variety of bone disorders by increasing bone mass to decrease fracture risk, but long-term use has been shown in some settings to compromise bone quality. Alternatively, Raloxifene (RAL) has recently been demonstrated to improve tissue quality and overall mechanical properties in a cell-independent manner by binding to collagen and increasing tissue hydration. We hypothesized that a combination of RAL and ZOL would improve mechanical and material properties of bone more than either monotherapy alone by enhancing both quantity and quality. In this study, wildtype (WT) and heterozygous (OIM+/-) male mice from the Osteogenesis Imperfecta (OI) murine model were treated with either RAL, ZOL, or both from 8 weeks to 16 weeks of age. Using the OIM model allows for investigation of therapeutic effects on a quality-based bone disease. Combination treatment resulted in higher trabecular architecture, cortical mechanical properties, and cortical fracture toughness in diseased mouse bone. Two fracture toughness properties, which are direct measures of the tissue's ability to resist the initiation and propagation of a crack, were significantly improved with combination treatment in OIM+/- compared to control. There was no significant effect on fracture toughness with either monotherapy alone in either genotype. Following the mass-based effects of ZOL, trabecular bone volume fraction was significantly higher with combination treatment in both genotypes. Combination treatment resulted in higher ultimate stress in both genotypes. RAL and combination treatment in OIM+/- also increased resilience compared to the control. In conclusion, this study demonstrates the beneficial effects of using combination drug treatments to increase bone mass while simultaneously improving tissue quality, especially to enhance the mechanical integrity of diseased bone. Combination therapies could be a potential method to improve bone health and combat skeletal fragility on both the microscopic and macroscopic levels.

Actomyosin Cortical Mechanical Properties in Nonadherent Cells Determined by Atomic Force Microscopy

The organization of filamentous actin and myosin II molecular motor contractility is known to modify the mechanical properties of the cell cortical actomyosin cytoskeleton. Here we describe a novel method, to our knowledge, for using force spectroscopy approach curves with tipless cantilevers to determine the actomyosin cortical tension, elastic modulus, and intracellular pressure of nonadherent cells. We validated the method by measuring the surface tension of water in oil microdrops deposited on a glass surface. We extracted an average tension of T ∼ 20.25 nN/μm, which agrees with macroscopic experimental methods. We then measured cortical mechanical properties in nonadherent human foreskin fibroblasts and THP-1 human monocytes before and after pharmacological perturbations of actomyosin activity. Our results show that myosin II activity and actin polymerization increase cortex tension and intracellular pressure, whereas branched actin networks decreased them. Interestingly, myosin II activity stiffens the cortex and branched actin networks soften it, but actin polymerization has no effect on cortex stiffness. Our method is capable of detecting changes in cell mechanical properties in response to perturbations of the cytoskeleton, allowing characterization with physically relevant parameters. Altogether, this simple method should be of broad application for deciphering the molecular regulation of cell cortical mechanical properties.

Structural and Mechanical Improvements to Bone Are Strain Dependent with Axial Compression of the Tibia in Female C57BL/6 Mice.

Strain-induced adaption of bone has been well-studied in an axial loading model of the mouse tibia. However, most outcomes of these studies are restricted to changes in bone architecture and do not explore the mechanical implications of those changes. Herein, we studied both the mechanical and morphological adaptions of bone to three strain levels using a targeted tibial loading mouse model. We hypothesized that loading would increase bone architecture and improve cortical mechanical properties in a dose-dependent fashion. The right tibiae of female C57BL/6 mice (8 week old) were compressively loaded for 2 weeks to a maximum compressive force of 8.8N, 10.6N, or 12.4N (generating periosteal strains on the anteromedial region of the mid-diaphysis of 1700 με, 2050 με, or 2400 με as determined by a strain calibration), while the left limb served as an non-loaded control. Following loading, ex vivo analyses of bone architecture and cortical mechanical integrity were assessed by micro-computed tomography and 4-point bending. Results indicated that loading improved bone architecture in a dose-dependent manner and improved mechanical outcomes at 2050 με. Loading to 2050 με resulted in a strong and compelling formation response in both cortical and cancellous regions. In addition, both structural and tissue level strength and energy dissipation were positively impacted in the diaphysis. Loading to the highest strain level also resulted in rapid and robust formation of bone in both cortical and cancellous regions. However, these improvements came at the cost of a woven bone response in half of the animals. Loading to the lowest strain level had little effect on bone architecture and failed to impact structural- or tissue-level mechanical properties. Potential systemic effects were identified for trabecular bone volume fraction, and in the pre-yield region of the force-displacement and stress-strain curves. Future studies will focus on a moderate load level which was largely beneficial in terms of cortical/cancellous structure and cortical mechanical function.

Cortical Actin Tension, Elastic Modulus and Cytosolic Pressure in Fibroblasts Determined using Atomic Force Microscopy

The cortical actin cytoskeleton lies just beneath the cell membrane to define cell shape and mechanical properties and thus plays a key role in cell biology processes such as migration and morphogenesis. The organization of actin filaments and actomyosin contractility are known to contribute to modifying the mechanical properties of the cortex. However, recent work report how these properties contribute to cortex tension and intracellular pressure. Here we propose a new method for using an atomic force microscope to determine actin cortex mechanical properties of non-adherent human foreskin fibroblasts including the cortex tension and intracellular pressure, but additionally, the cortex elastic modulus which has not been measured before. First, we validated the method by measuring the surface tension of water-in-oil microdrops deposited on a glass surface. We extracted an average tension of T∼20.2 nN/µm, which agrees with macroscopic experimental methods. We then proceeded to measure cortical actin mechanical properties in non-adherent fibroblasts, and compare this to the properties after inducing two perturbations (i) adding blebbistatin which inhibits myosin II molecular motor activity, and (ii) adding CK-666 which inhibits Arp2/3-mediated actin branching. Our results show that perturbing the actin cortex had significant changes in each of the cortical mechanical properties: blebbistatin reduced them by ∼50%, while CK-666 increased them by ∼2-fold. These results validate our novel method for determining the quantitative mechanics of the actin cortex in eukaryotic cells.

Biomechanical characteristics of regenerated cortical bone in the canine mandible

To test the mechanical properties of regenerate cortical bone created using mandibular bone transport (MBT) distraction, five adult male American foxhound dogs underwent unilateral distraction of the mandible with a novel MBT device placed to linearly repair a 30-35 mm bone defect. The animals were sacrificed 12 weeks after the beginning of the consolidation period. Fourteen cylindrical specimens were taken from the inner (lingual) and outer (buccal) plates of the reconstructed mandible and 21 control specimens were removed from the contralateral aspect of the mandible. The mechanical properties of the 35 cylindrical cortical bone specimens were assessed by using a non-destructive pulse ultrasound technique. Results showed that all of the cortical mechanical properties exhibit higher numerical values on the control side than the MBT regenerate side. In addition, both densities and the elastic moduli in the direction of maximum stiffness of the regenerate cortical bone specimens are higher on the lingual side than the buccal side. Interestingly, there is no statistical difference between elastic modulus (E(1) and E(2) ) in orthogonal directions throughout the 35 cortical specimens. The data suggest that not only is the regenerate canine cortical bone heterogeneous, but the elastic mechanical properties tend to approximate transverse isotropy at a tissue level, as opposed to control cortical bone, which is orthotropic. In addition, the elastic mechanical properties are higher not only on the control side but also in the lingual anatomical position, suggesting a stress shielding effect from the presence of the reconstruction plate.

Actin and Myosin-II Regulate Cortical Mechanical Properties During Mouse Oocyte Meiotic Maturation.

Actin and Myosin-II Regulate Cortical Mechanical Properties During Mouse Oocyte Meiotic Maturation.

Prediction of mechanical properties of cortical bone by quantitative computed tomography

The relevance of Finite-Element models for hip fracture prediction should be increased by the recent subject-specific methods based on computed tomography (CT-scan), regarding the geometry as well as the material properties. The present study focused on the prediction of subject-specific mechanical parameters of cortical bone (Young's modulus and ultimate strength) from the bone density measured by CT. A total of 46 compression and 46 tension samples from 13 donors (mean age ± S.D.: 81.8 ± 12.7 years) were harvested in the femoral mid-diaphysis and tested until failure. The Young's modulus and ultimate strength were linearly correlated with the bone density measured by CT, for tension as well as compression (0.43 < r 2 < 0.72, p < 0.001). To take into account the remaining uncertainties on the mechanical properties prediction, the standard error of the estimate (S.E.E.) was evaluated in each case (2694–2788 MPa for Young's modulus, 13–16 MPa for ultimate strength). The significant correlations obtained in the present study and the quantification of the errors will be helpful for the assessment of the cortical mechanical properties from the CT-scan data in order to create subject-specific FE-models.

High-frequency oscillatory motions enhance the simulated mechanical properties of non-weight bearing trabecular bone

Extremely low-level oscillatory accelerations, applied without constraint, can increase bone formation. Here, we tested the hypothesis that high-frequency oscillations, applied in the absence of functional weight bearing, can be sensed by trabecular bone to produce a structure that is more efficient in sustaining applied loads. The left leg of anesthetized adult female mice ( n=18) was subjected to high-frequency oscillations at 45 Hz, 0.6 g for 20 min/day, 5 days/week for 3 weeks, while the contralateral leg served as an internal control. To remove the potential interference of the habitual strain environment with the imposed physical signal, the hindlimbs of these mice were chronically unloaded. In vivo microCT scans of the proximal metaphyseal region of the tibia were transformed into finite element meshes to evaluate trabecular and cortical mechanical properties. Simulated longitudinal compression tests showed that the short applications of high-frequency oscillations were sensed primarily by trabecular bone. At the end of the experimental period, apparent trabecular stiffness of the oscillated bones was 38% ( p<0.001) greater than that of non-weight bearing controls. Simulated uniaxial loads applied to trabecular bone induced 21%, 52%, and 131% greater ( p<0.05) median, peak compressive, and peak tensile longitudinal stresses in control than in stimulated bones. Non-weight bearing control bones were also characterized by greater transverse normal and shear stresses (77% and 54%, respectively, p<0.001) as well as 35% greater ( p=0.03) longitudinal shear stresses. Compared to normal age-matched controls ( n=18), oscillations were able to attenuate, but not fully prevent, the decline in trabecular mechanical properties associated with the removal of weight bearing. These data indicate not only that bone cells can sense low-level, high-frequency oscillatory accelerations, but also that they can orchestrate a structural response that produces a stiffer trabecular structure that may be less prone to fracture.

Long-Term Disuse Osteoporosis Seems Less Sensitive to Bisphosphonate Treatment Than Other Osteoporosis

We sought to determine whether risedronate can preserve cortical bone mass and mechanical properties during long-term disuse in dogs, assessed by histomorphometry and biomechanics on metacarpal diaphyses. Risedronate slowed cortical thinning and partially preserved mechanical properties, but it was unable to suppress bone loss to the degree seen in other osteoporoses. Disuse induces dramatic bone loss resulting from greatly elevated osteoclastic resorption. Targeting osteoclasts with antiresorptive agents, such as bisphosphonates, should be an effective countermeasure for preventing disuse osteoporosis. Single forelimbs from beagles (5-7 years old, n = 28) were immobilized (IM) for 12 months. Age-matched, non-IM dogs served as controls. One-half the animals received either risedronate (RIS, 1 mg/kg) or vehicle daily. Histomorphometry was performed on second metacarpal mid-diaphyses. Cortical mechanical properties were determined by testing third metacarpal diaphyses in four-point bending. IM caused marked reduction in cortical area (-42%) and cortical thinning (-40%) through endocortical resorption, extensive intracortical tunneling, and periosteal resorption; both bone resorption and formation were significantly elevated over control levels on all envelopes. IM also decreased maximum load and stiffness by approximately 80% compared with controls. RIS reduced both periosteal bone loss and marrow cavity expansion; however, cortical area remained significantly lower in RIS-treated IM animals than in untreated non-IM controls (-16%). RIS also increased resorption indices in all envelopes compared with nontreated IM, indicating that RIS suppressed osteoclast activity but not osteoclast recruitment. RIS did not affect bone formation. RIS treatment conserved some whole bone mechanical properties, but they were still significantly lower than in controls. There were no significant differences in tissue level material properties among the groups. RIS treatment reduces cortical bone loss at periosteal and endocortical surfaces caused by long-term immobilization, thus partially conserving tissue mechanical properties. This modest effect contrasts with more dramatic actions of the bisphosphonate in other osteoporoses. Our results suggest that risedronate impairs osteoclastic function but cannot completely overcome the intense stimulus for osteoclast recruitment during prolonged disuse.

Dynacortin contributes to cortical viscoelasticity and helps define the shape changes of cytokinesis.

During cytokinesis, global and equatorial pathways deform the cell cortex in a stereotypical manner, which leads to daughter cell separation. Equatorial forces are largely generated by myosin-II and the actin crosslinker, cortexillin-I. In contrast, global mechanics are determined by the cortical cytoskeleton, including the actin crosslinker, dynacortin. We used direct morphometric characterization and laser-tracking microrheology to quantify cortical mechanical properties of wild-type and cortexillin-I and dynacortin mutant Dictyostelium cells. Both cortexillin-I and dynacortin influence cytokinesis and interphase cortical viscoelasticity as predicted from genetics and biochemical data using purified dynacortin proteins. Our studies suggest that the regulation of cytokinesis ultimately requires modulation of proteins that control the cortical mechanical properties that establish the force-balance that specifies the shapes of cytokinesis. The combination of genetic, biochemical, and biophysical observations suggests that the cell's cortical mechanical properties control how the cortex is remodeled during cytokinesis.

Cancellous and cortical bone mechanical properties and tissue dynamics during pregnancy, lactation, and postlactation in the rat.

There are substantial changes in maternal skeletal dynamics during pregnancy, lactation, and after lactation. The purpose of this study was to correlate changes in cortical and cancellous bone mass, structure, and dynamics with mechanical properties during and after the first reproductive cycle in rats. Rats were mated and groups were taken at parturition, end of lactation and 8 wk after weaning, and were compared with age-matched, nulliparous controls. Measurements were taken on femoral cortical bone and lumbar vertebral body cancellous bone. At the end of pregnancy, there was an increase in cortical periosteal bone formation and an increase in cortical volume, but a suppression of turnover in cancellous bone with no change in cancellous or cortical mechanical properties. Lactation was associated with a decrease in cortical and cancellous bone strength with a decrease in bone volume, but an increase in turnover on cancellous and endocortical surfaces. After lactation, there was a partial or full restoration of mechanical properties. This study demonstrates substantial changes in bone mechanics that correlate with changes in bone structure and dynamics during the first reproductive cycle in rats. The greatest changes were observed during the lactation period with partial or full recovery in the postlactational period.

Conservation of the elastic and flexural moduli of osteopenic femoral cortical bone in experimental inflammatory arthritis in the rabbit.

Experimental inflammatory arthritis (EIA) produced by carrageenan injection provokes a rapid bone remodeling state with cortical and cancellous bone loss. The objective of this study was to determine whether changes in cortical mechanical properties and/or geometry occur in long bones, either near or remote to the site of inflammation. EIA was induced in the right tibio-femoral joint of rabbits over 56 days. The right humerus and right femur from 15 normal and 25 arthritis group animals were excized. Semi-cylindrical specimens of the medial cortical shaft were subjected to non-destructive four-point bending tests. Transverse sections at the four contact sites of the loading jig were photographed and digitized to obtain average cross-sectional area (A) and moment of inertia (I). Moment of inertia and slope of the load/deflection curve permitted calculation of modulus of elasticity (E) for each specimen. Load/time curves were also used to calculate per cent stress remaining in relaxation experiments. Per cent stress remaining, E, A, I and square root of I/A (radius of gyration) were examined for differences by bone (humerus, femur) and by treatment (N,A) using two way ANOVA. The induction of inflammatory arthritis did not significantly alter the modulus of elasticity in either the femur or humerus; however, arthritis reduced the moment of inertia from 34.54 +/- 2.88 x 10(-12) m4 to 25.06+/-1.80 x 10(-12) m4 (mean+/-SEM, p < 0.05). This was observed in the femur (near the arthritic joint), but not in the humerus (remote from arthritic joint). Analysis of area and ratio I/A demonstrated that this geometric effect of treatment was due to reduced area without gross cross-sectional shape changes. Per cent stress remaining in the femur (but not in the humerus) was higher in the arthritis specimens than in the normal specimens (N: 80.86 < 0.97%; A: 83.25 +/- 0.71%, p < 0.05). Thus, in this arthritis model, the principal mechanical or geometric effect on cortical bone was reduction of the cross-sectional area and moment of inertia. The viscoelastic relaxation response of bone was also altered, perhaps due to loss of water or collagen degradation.